Canting formula

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Gustavo, May 3, 2006.

  1. Gustavo

    Gustavo Writers Guild

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    Hi there,

    I'm trying to build an easy Excel-based spreadsheet, to account for CANT.

    So far, I've not found an easy or accurate CANTING equuation

    Please, can you help me on this?

    In short, if someone could post an ACCURATE CANT FORMULA my very best thank you!

    regards, Gustavo
     
  2. Buffalobob

    Buffalobob Writers Guild

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    Are you asking about when the scope is so many degrees off of vertical over the barrel. In other words when the middle of the scope is not in direct vertical line with the center of the barrel?


    To do the calculation you need to know the center of the scope height above the center of the barrel. You would then use the angle of the cant and either a sine angle times radius (scope height originally) and then against the internal cross hair adjustments. To get the windage error you would use a cosine to determine the effect.

    you will have an error in the windage adjustments also and will need to make a similar but reverse calculation. Error may be additive or not depending on up or down and right versus left and right and which side the cant is on.

    First verify what your question really is because it is a lot of work to build such a spread sheet as I have described.



    I may have this backward because I have not gotten out a peice of paper and a penicl to draw out the angles and verify sine versus cosine and am just working with mental pictures.
     

  3. Bart B

    Bart B Well-Known Member

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    Hey, Gustavo, my wife spent a couple of nights in Buenos Aires a couple of years ago on her way to Ushuaia near Cape Horn. Beautiful city, she said and really neat and nice people!!!!

    This cant stuff is a simple situation. I've been dealing with it for years.

    Example: at 1000 yards, bullet drop is 350 inches and the cant angle is 1 degree.

    Horizontal impact change due to cant is equal to bullet drop at target range multiplied by the sine of the angle. Sine of 1 degree is 0.01745, that times 350 equals 6.11 inches horizontal impact change.

    Vertical impact change equals bullet drop minus the cosine of the angle multiplied by the bullet drop. Cosine of 1 degree is 0.99985, that times 350 equals 349.95 inches, and 350 minus 349.95 equals .05 inches below the horizontal where the bullet would strike.

    Horizontal change is the worst problem. Even with a 10 degree cant in the above example, the bullet would strike about 60 inches to the side but only about 5 inches low. Sight height above the bore doesn't matter.
     
  4. Eaglet

    Eaglet Well-Known Member

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  5. Buffalobob

    Buffalobob Writers Guild

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    Good article, Eaglet.
     
  6. Bart B

    Bart B Well-Known Member

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    [ QUOTE ]
    Here is some good reading on it.
    http://www.riflescopelevel.com/cant_errors.html#vha

    [/ QUOTE ]Sad to say, the data in this link is wrong because it's incomplete. It uses the high point in the trajectory to the target. That's not where the bullet strikes; the bullet strikes the target. Which is why one has to use bullet drop at the target range. The horizontal change in impact will be more than were the trajectory's high point (THP) is horizontlly because bullet drop at the THP is not quite half of the drop at the target. In the link's diagram, bullet impact will be further to the right of the indicated circle and about as far below the line of sight as the canted THP circle is below the original THP circle.

    I've heard or read this link's explanation of cant for over 40 years. This one goes only half way showing what happens about 60% of the distance to the target. It would be correct if it showed the impact change at the target 'cause that's what matters and that's where the bullet drop number is important.
     
  7. Buffalobob

    Buffalobob Writers Guild

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    /ubbthreads/images/graemlins/shocked.gif
     
  8. Jimm

    Jimm Writers Guild

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    Eaglet ,

    Thanks for the article , Jim B.
     
  9. Brown Dog

    Brown Dog Writers Guild

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    Bart,

    Well, a quiet afternoon at work became occupied with trigonometry revision!

    I’d never even thought about quantifying this before coming across this thread. What you say makes total sense, but I just about burst my brain duplicating your figures…..couldn’t remember the 1st formula at all…!

    ..it was quiet enough that I plotted it all out on a ppt slide (If this helps anyone else? /ubbthreads/images/graemlins/crazy.gif /ubbthreads/images/graemlins/crazy.gif /ubbthreads/images/graemlins/smile.gif!):

    (the left hand red line is the uncanted drop, the right hand is the canted drop)
    [​IMG]
    [​IMG]

    .......I have to sleep now /ubbthreads/images/graemlins/smile.gif.
     
  10. Gustavo

    Gustavo Writers Guild

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    Thanks you very much! very good ansers to let me think more than I care about the issue!

    regards, Gustavo
     
  11. Bart B

    Bart B Well-Known Member

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    [ QUOTE ]
    Well, a quiet afternoon at work became occupied with trigonometry revision!

    [/ QUOTE ]I know; oft times I've applied grade school trig to shooting situations. It sure puts some myths in the "busted" category.

    Your graphic illustration of my example is great. I'll send you three "attaboys" just as soon as I figure out how to ship them.
     
  12. Gustavo

    Gustavo Writers Guild

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    [ QUOTE ]
    Hey, Gustavo, my wife spent a couple of nights in Buenos Aires a couple of years ago on her way to Ushuaia near Cape Horn. Beautiful city, she said and really neat and nice people!!!!

    [/ QUOTE ]

    Hi Bart,

    Nice to hear that your wife enjoyed her stay in my country and that she visited the beautiful Ushuahia! (world's southernmost city)

    Did she sail around Cape Horn?? (Cabo de Hornos, which is the spanish name and does not translate into Cape Horn = Cabo de Cuerno ??)

    Hope, that you and other members of this forum, can someday take a trip donw here to really enjoy Patagonia, where I'm sure if they want LRH...will have plenty for sure!!

    I guarantee WIND(lots of it) AND LONG SHOTS (the norm)

    On the other hand, thank your your time in giving a neat answer to my issue

    regards, Gustavo